Microchannel heat exchangers are currently designed in a parallel flow configuration, wherein there is a long inlet header that extends the length of the core and feeds multiple parallel tubes that then feed into an outlet header. The diameter of the headers must be larger than the major axis of the microchannel tube. When this parallel flow microchannel heat exchanger operates as an evaporator, two-phase refrigerant is being fed into the inlet header. Since this two-phase refrigerant is a mixture of vapor and liquid, it tends to separate in the inlet header leading to maldistribution within the evaporator (i.e. some tubes are fed mostly vapor instead of a balanced mixture of vapor and liquid), which has a negative effect on the cooling capacity and efficiency of the air conditioner. Because the performance is compromised in this manner, additional surface must be added to the evaporator to match the capacity and efficiency of a comparable round tube, plate fin evaporator. This increases the cost as well.
Typically, an inlet header is only fed from one side in what is referred to as a direct feed approach. Such a direct feed approach causes two-phase refrigerant to flow through the entire length of the header, with the vapor and liquid tending to separate out such that some tubes get mostly vapor and others get mostly liquid, thereby resulting in dry surfaces and poor utilization of the heat exchanger.
An alternative to the direct feed approach is to use a distributor leading to multiple feeder tubes that feed into baffled sections of the header. This method results in considerable additional expense over the direct feed method as additional hardware such as the distributor/feeder tube assembly must be added as well as the baffles in the header.
When particular structures are added to heat exchangers in order to promote uniform flow from the inlet manifold to the microchannels during cooling mode operation, those same structures may interfere with refrigerant flowing in the opposite direction during operation in the heating mode.